Trans- isomer Subject

Butadiene, the simplest conjugated diene, has been the subject of intensive theoretical and experimental studies to understand its physical and chemical properties. The conjugation of the double bonds makes it 15 kJ/mole (3.6 kcal/mol) (13) more thermodynamically stable than a molecule with two isolated single bonds. The r-trans isomer, often called the trans form, is more stable than the s-cis form at room temperature. Although there is a 20 kJ/mole (4.8 kcal/mol) rotational barrier (14,15), rapid equiUbrium allows reactions to take place with either the s-cis or r-trans form (16,17). 

The second subject is concerned with cis and irans isomers. The trans isomer has the higher value (except for azobenzene) and the larger max.- This win be apparent from the data in Table XIII. 

There has been only one report of industrial poisoning, a fatality caused by very high vapor inhalation in a small enclosure. The isomeric concentration of the vapor was not reported, nor were the level and duration of the exposure or symptoms of toxicity. In another early report, exposure to the trans isomer at 2200ppm caused nausea, drowsiness, fatigue, vertigo, and increased intracranial pressure in two human subjects. ... 

Several cyclodipeptides have been subjected to base-catalyzed epimerization (EtOH/NaOEt at 30-75°C) and the ratio of cis-to-trans isomers at equilibrium has been determined (74JA3985). The results have been correlated with the conformation of the molecules. Thus, cyclo(Pro-Pro) NMR studies (73JA6142) have indicated a boat form in the cis and a planar form in the trans diastereomer. In the latter, the pyrrolidine rings take up a half-chair conformation, which is greatly strained as long as the amide bonds are planar. This renders the trans less stable than the cis diastereomer. Consequently, at equilibrium, only cis diastereomer is found the trans isomer occurs to the extent of less than 0.5%. 

Photoexcitation, in orbital terms, can be described as the promotion of a single electron from HOMO to LUMO by the interaction with light. When either transox cw-2-butene is subjected to UV light, the same mixture of cis and trans isomers results. Explain. 

Truce and collaborators (7-9) have shown that cis-dichloroethylene 20 reacts readily with sodium -toluenethiolate in the presence of sodium ethoxide to give cis-1,2-bis-p-tolylmercapto-ethylene 22 while the trans-isomer 21, when subjected to the same conditions is recovered unchanged. Convincing evidence was obtained that the conversion 20 22 takes place via the intermediates 23, 24, and 25. Truce and Simms (9) have also observed that the base-catalyzed addition of -toluenethiol to phenyl acetylene and to 2-butyne yields ci s-styryl -tolylsulfide (26) and 2-p-tolylmercapto-trans-2-butene (27) respectively. 

The smaller-sized cyclohexene and cycloheptene have also been subjected to enantiodifferentiating photoisomerization, although the corresponding ( )-isomers are short-lived transient species. Photosensitization of (Z)-cyclohexene 31Z with chiral benzene(poly)carboxylates affords trans-anti-trans-, cis-trans-, and cis-anti-cis-cyclodimers 34 (Sch. 32). Interestingly, of the former two chiral products, only the trans-anti-trans isomer is optically active and its ee reaches up to 68%, whilst the cis-trans isomer is totally racemic under a variety of irradiation conditions, for which two competing, concerted, and stepwise cyclodimerization mechanisms are responsible. Thus, the enantiodifferentiating photoisomerization of 31Z to the optically... 

A second important reaction of L-AuR species is the oxidative addition (see Oxidative Addition) of halogen or alkyl halides, which leads, at least in a first step, to organo(dihalo)gold(IIl) or diorgano(halo)gold(III) products. The structures are transformed from linear to square planar, and therefore cis and trans isomers are possible. The products may undergo secondary reactions and/or become subject to a reductive elimination (see Reductive Elimination) of other substituent combinations (equations 41 and 42). ... 

Dideoxynucleosides show potent anti-retroviral activity against HIV-specific reverse transcriptase80-83. In particular, 2, 3 -dideoxy-3 -C-cyano-2 -substituted thymidine derivatives (33 A and 33 B) with a free 5 -hydroxy function (R1 = H) are potential inhibitors of the HIV-reverse transcriptase-promoted c-DNA synthesis. As these compounds have yet to be prepared by another method, the 3 -ene-nitrile 3284 was subjected to conjugate addition reactions with ammonia, primary amines, secondary amines and carbon nucleophiles. Most of these nucleophilic amine addition reactions give either the trans-isomer 33 A as the sole product (e.g., reaction with pyrrolidine, piperidine, morpholine), or as the major product along with the c/s-isomer (e.g., reaction with methylamine, benzylamine), except for the reaction with ammonia where the cts-isomer 33B is formed as the major product84. 

Recrystallization apparently leads to considerable loss of material without appreciable gain in purity. From a 5.1-g. sample of the crude mixture subjected to both chromatography and recrystallization the checkers isolated 0.72 g. of cis isomer, m.p. 163.4 to 164.0 , and 2.25 g. of trans isomer, m.p. 169.2 to 169.6°. 

When chloromethyl phenyl sulfone (130 Ar = phenyl) was subjected to the Darzens-type reaction with an aldehyde, a thermodynamically stable trans isomer (133) was produced exclusively (equation 32). This is in sharp contrast with the corresponding reaction of chloromethyl phenyl sulfoxide. Tavares proposed that the initial nucleophilic attack of the a-sulfonyl carbanion upon a carbonyl compound is rapidly reversible due to its stability, and that the product-determining step is the ring closure. Thermodynamic equilibrium between the two diastereomers of (132) allows predominant formation of the thermodynamically stable isomer (133) from the preferred transition state. ... 

In analogy to the preparation of bicyclic triene 37 from bisacetylene J449, the diethynyloxiranes 35 and thiiranes 36 (both cis and trans isomers can be employed tram- 36 leads to desulfurized product only) were subjected to gas phase flow-pyrolyses to effect clean, though low yielding, rearrangement to furo- and thienocyclobutadiene J S50 and 39sl respectively, systems that are isoelectronic with anion 33. Both 38 and 39 are planar Sir-systems and as such exhibit typical... 

Treatment of A-nitroso-iV-(cr5-2-phenylcyclopropyl)urea with excess sodium formate in methanol at 25 "C for 15 hours affords ( )-3-methoxy-l-phenylprop-l-ene (25%) and 3-methoxy-3-phenylprop-l-ene (60%, Table 15, entry 7). An almost identical result is obtained when the tran -isomer is used as substrate. Formation of the 2-phenylcyclopropyldiazonium ion, decomposition with loss of nitrogen to the 2-phenylcyclopropyl cation, followed by ring opening to form the phenylallyl cation, can explain the result. When [l- Hi]aminocyclopropane is subjected to deamination with nitrous acid in water at 0°C (Table 15, entry 8), allyl alcohol is obtained, which is isolated as the 4-(phenylazo)benzoate derivative in 44% yield. In this case, the deuterium is only located at the C2 allyl position. ... 

Similarly, the diethyl 2-(3-oxocyclopentyl)- and 2-(4-oxocyclohexyl)ethylphosphonates have been subjected to the standard Bucherer-Berg reaction conditions [KCN, (NH4)2CO, NH4CI in EtOH-HjO at 60°C] to give spirohydantoins, which are isolated as a mixture of cis- and trans-isomers. Hydrolysis of hydantoins needs heating at 150°C in a sealed tube with concentrated HCl to provide the amino acid hydrochlorides. ... 

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